1. Field of the Invention
The present invention relates to systems and methods for determining the angular position of a rotor. More specifically, the present invention relates to systems and methods for digital signal processing of gimbal shaft resolver rotor angle signals in real-time control systems.
2. Description of the Related Art
A resolver is a device used to sense the angular position of a rotatable member, such as a shaft. In the prior art, resolvers were used with a tracking converter circuit to monitor the angular position of rotor shafts in real time control systems. For example, such circuits were used to sense the shaft angle of pitch and yaw gimbal in guided missile systems.
A resolver typically includes a rotor which has an electric coil winding that is excited by a sinusoidal oscillating reference voltage. The resolver also includes two stator electric coil windings which are positioned orthogonal with respect to one another. Because of the proximal and orthogonal relationship of stator winding with rotor winding the two stator windings are excited by electromagnetic coupling from the rotor winding and produce output signals which include components of the reference oscillator signal and the sine and cosine of the angle of rotation. Therefore, one winding is called the xe2x80x98sinxe2x80x99 winding and the other is called the xe2x80x98cosxe2x80x99 winding. When the rotor winding is aligned with the cos winding, then the gimbal angle is zero.
The analog output signals from stator windings are applied to the reference inputs of a pair of digital to analog converters. A digital input to the digital to analog converters comes from and up-down counter which represents a trial angle used in tracking the actual rotor angle. Thus, the analog outputs of the two digital to analog converters include components of the reference oscillator, the sine and cosine of the rotor angle, and the cosine and sine of the trial angle, respectively.
The analog output signals of the digital to analog converters are fed into the normal and inverted inputs of an error amplifier and are thus subtracted to create an error signal which is output and contains components of the reference oscillator signal and the sine of the difference between the rotor angle and the trial angle. The error signal is further demodulated by a phase sensitive detector which outputs a direct current error signal that is proportional to just the sine of the difference between the rotor angle and trial angle.
The error signal is subsequently fed to a frequency shaping integrator whose output drives the input of a voltage controlled oscillator which in turn is applied to, the input of the up-down counter. As noted before, the UP-DOWN counter output drives the digital to analog converters such that the error signal is driven to zero due to the feedback loop design of this tracking converter circuit. The rotor, or gimbal, angle G is represented in binary form as the count of up-down counter when the difference between trial angle P and gimbal angle G are driven to zero. Therefore, the rotor angle is output in digital form from UP-DOWN counter which is used by subsequent circuitry in a typical control system.
In a typical application, such as a missile guidance system, the reference oscillator will be in the frequency range of 1 kHz to 10 kHz and the rotor angle will represent that gimbal shaft position, and will be sampled by reading the output of the up-down counter in the range of 1000 to 10,000 times per second, or approximately once for every cycle of the reference voltage. Tracking converters are well understood and have good performance in that the rotor shaft angle can be reliably read as frequently as once per cycle of the reference oscillator as long as the tracking converter error is nulled.
In a real world environment, the tracking converter will be subjected to noise of various kinds. In the example of a missile guidance system, electromagnetic noise is generated within the missile from other internal systems which may include motors and power supplier, or may come from sources outside the missile. As the accuracy of the rotor angle sensing is critical to guidance, in that the rotor represents the angle of the yaw and pitch gimbal for example, which are fed to navigation and guidance systems for the missile, it is essential that the rotor angle be accurately measured. This requires a design with good noise immunity because if noise exceeds the system immunity of the tracking converter, the rotor angle, or gimbal position, fed to other systems will not be accurate leading to improper navigation and guidance of the missile.
The prior art tracking converters offer good noise immunity because the system doubly integrates the signal. First, it is integrated in the frequency shaping integrator and then by up-down counter. This effectively averages the wave form over many periods of the reference voltage wave form. Since inductively coupled noise spikes typically have equal positive and negative going wave forms, such noise is well suppressed through integration. Noise immunity is also augmented by rejection of frequencies beyond the reference frequency by the phase detector, which is a narrow band device that inherently suppresses wide band noise.
The conventional tracking converter, while effective and reliable at tracking a resolver rotor angle, such as a gimbal shaft angle in a guided missile, is problematic because of cost. The digital to analog converters must be high performance devices and are often available only from a single source because of their highly specialized nature. In addition, there are many components needed for the other circuit devices. In most applications of rotor angle position sensing technology, it is necessary to sense a plurality of rotor angle positions simultaneously, so multiple resolvers are required. For example, in a guided missile, there would be both a yaw and pitch gimbal shafts to sense. This would then require a separate instance of the aforementioned tracking converter circuit for each gimbal axis utilized in the missile, which multiplies the cost factor.
Thus there is a need in the art for a tracking circuit design that reduces component cost, allows for measuring more than one resolver rotor angle, and provides adequate noise immunity, measuring accuracy, and performance to equal or exceed, the prior art tracking converters.
The need in the art is addressed by the apparatus and methods of the present invention. The illustrative embodiments of the inventive apparatus are circuits in which a resolver is coupled in circuit to a digital signal processor that samples the resolver stator winding signals and calculates the rotor angle. The present invention applies digital signaling processing to the sampled signals in order to determine the rotor angle. However, since digital signaling processing is inherently a sampling process, it is sensitive to noise. In addition to the aforementioned cost savings, the digital signaling techniques of the present invention advantageously improve the computation of the angle in the presence of noise. In a first illustrative embodiment, an over sampling technique is employed. In a second illustrative embodiment, a fast Fourier transform technique is employed. In a third illustrative embodiment a sigma-delta converter is employed in conjunction with the digital signal processor.
In the first illustrative embodiment, the apparatus for measuring angular position of a resolver rotor includes a resolver which has a rotor with a winding excited by a periodic wave reference voltage. The resolver also has a first stator winding, excited by energy coupled from the rotor winding, and a second stator winding, also excited by energy coupled from the rotor winding. The two stator windings are oriented orthogonal with respect to each other. The apparatus includes a first and second low pass filter with corner frequencies substantially higher than the frequency of the periodic wave reference voltage. The stator windings are coupled through the low pass filters. The apparatus also has a multiplexer with a first, second, and perhaps more, inputs coupled to the outputs of the two low pass filters. The multiplexer has a control input for selecting one of its inputs to be coupled to an output of the multiplexer. An analog to digital converter receives the output of the multiplexer and converts the analog resolver stator winding signals to digital signals. The digital signals are coupled to a digital signal processor. The digital signal processor also has a control output coupled to the control input of the multiplexer for selecting which one of the multiplexer inputs is coupled through at any given time. The digital signal processor repeatedly samples the output of the analog to digital converter and accumulates a plurality of data points representative of the output voltage present on the stator windings. The digital signal processor also calculates the rotor angle by averaging several representative numeric values derived from the plurality of data points.
In a second illustrative embodiment, the apparatus for measuring angular position of a resolver rotor includes a resolver which has a rotor with a winding excited by a periodic wave reference voltage. The resolver also has a first stator winding, excited by energy coupled from the rotor winding, and a second stator winding, also excited by energy coupled from the rotor winding. The two stator windings are oriented orthogonal with respect to each other. The apparatus includes a first and second low pass filter with corner frequencies substantially higher than the frequency of the periodic wave reference voltage. The stator windings are coupled through the low pass filters. The apparatus also has a multiplexer with a first, second, and perhaps more, inputs coupled to the outputs of the two low pass filters. The multiplexer has a control input for selecting one of its inputs to be coupled to an output of the multiplexer. An analog to digital converter receives the output of the multiplexer and converts the analog resolver stator winding signals to digital signals. The digital signals are coupled to a digital signal processor. The digital signal processor also has a control output coupled to the control input of the multiplexer for selecting which one of the multiplexer inputs is coupled through at any given time. The digital signal processor samples the output of the analog to digital converter and accumulates a plurality of data points representative of the output voltage present on the stator windings. The digital signal processor calculates a series of spectral coefficients using a fast Fourier transform algorithm applied to the plurality of data points thereby isolating the majority of noise energy from the desired signal energy by selecting a group of pertinent Fourier coefficients, and subsequently calculates the rotor angle using the pertinent Fourier coefficients.
In a third illustrative embodiment, the apparatus for measuring angular position of a resolver rotor includes a resolver which has a rotor with a winding excited by a periodic wave reference voltage. The resolver also has a first stator winding, excited by energy coupled from the rotor winding, and a second stator winding, also excited by energy coupled from the rotor winding. The two stator windings are oriented orthogonal with respect to each other. The apparatus includes a first and second low pass filter with corner frequencies substantially higher than the frequency of the periodic wave reference voltage. The stator windings are coupled through the low pass filters. Two sigma-delta converters are coupled to the outputs of the low pass filters, in place of the analog to digital converters used in the previous embodiment. A digital signal processor is coupled to the outputs of the sigma-delta converters. The digital signal processor samples said outputs of the sigma-delta converters and accumulates a plurality of data points representative of the output voltage present on the outputs of the stator windings. The digital signal processor calculates the rotor angle by averaging a plurality of resultant values derived from the plurality of data points.